1. Field of the Invention
The present invention relates to restoring failed communication links in a communication network. More particularly, the present invention relates to a system and method for restoring communication in an optical domain of a communication network.
2. Related Art
Current restoration techniques for restoring communication traffic interrupted by a break or other failure in a fiber optic cable use electronic cross-connects for provisioning spare capacity. These techniques are extremely expensive and complex.
FIG. 1 shows a block diagram of an exemplary communication network 10 having at least optical three domains, or sub-networks, 12-14 shown as contiguous rings. Domains 12-14 operate independently from each other and are connected together by nodes or gateways 15-19, which are usually electrical cross-connects. As shown in FIG. 1, domain 12 is connected to a domain (not shown) through gateway 15, to domain 13 through gateway 17, and to domain 14 through gateway 16. Domain 13 is connected to domain 14 through gateway 18 and to a domain (not shown) through gateway 19. Domains 12-14 may also include other gateways, which are not also not shown. In the present example, communication traffic 11 traverses domains 12 and 13.
FIG. 2 shows a situation in which an optical fiber of domain 13 has been cut at 20 between gateways 17 and 19 by construction equipment 21. Traffic 11 is restored, however, through gateways 18 and 19.
FIG. 3 shows a simplified hierarchical network model 30 of a typical multi-layer restoration process for a communication network which can be used for restoring communication traffic 11 through network 10. The left-most column of blocks of FIG. 3 represents layers of the network. The center column of blocks represents the particular restoration process used at each corresponding network layer. The right-most column of blocks sets forth the alarm and database requirements at each corresponding network layer.
Each of the physical, logical and service layers 31, 34, and 37 of network model 30 contributes to the overall restoration process for maximizing use of expensive spare transmission capacity of the network. At the physical layer 31, the restoration process is a transmission-based restoration process 32, so named because it is activated by transmission alarms. At this level of the restoration process, the physical layer receives rerouting instructions from a transmission topology database (see block 33).
If the physical layer 31 lacks sufficient capacity for restoring all circuits affected by a failure, the restoration process proceeds to the logical layer 34 of model 30. At the logical layer, restoration is a circuit-based restoration process 35 activated by circuit alarms that searches for individual spare circuits which can be linked together based on a circuit database for restoring additional traffic around the fiber cut (see block 36).
If there are any remaining circuits that have not been restored by the first two layers of model 30, restoration proceeds to service layer 37 for an application-based restoration process 38. Application alarms are activated and the remaining circuits are rerouted by the application-based restoration process based on a network topology database (see block 39).
It is desired that communication traffic 11 be automatically restored within the domain having a failure so that the respective domains remain completely independent from each other. This results in network management being simplified by analysis of alarm information occurring only within the domain of the failure for determining the location of the fiber cut, and by not allowing circuit level alarms resulting from the fiber cut to permeate through the entire network 10.
Presently, electrical cross-connects are used at the gateways of the domains as the principle switching components for network restoration. If every circuit within a domain, whether traffic bearing or spare, is to be restored within the domain in which there is a failure, every circuit must enter and exit the cross-connects. This means that an electrical cross-connect restoration system has a disadvantage of a centralized network management system that requires full knowledge of every circuit on the network for automatically "building" restoration routes. Thus, a large accurate database must be maintained and searched each time a restoration process is activated. Future broadband services further complicate these issues by requiring concatenation instructions and knowledge mapping formats in addition to knowledge of the location of each circuit.
Additionally, since the cross-connect restoration process is circuit-based, circuit alarms are used for indicating that a failure has occurred. Given a large number of circuits on a network and that many events other than fiber cuts cause circuit alarms, such as maintenance procedures, circuit-based restoration systems require a complex network management analysis engine. Such a complex network management analysis engine must be capable of determining exactly where restoration capacity is required and separating actual outages from day-to-day events that can also cause circuit alarms.
Use of electrical cross-connects also precludes use of express optical pipes and, in general, makes the domain architecture unacceptable because of electrical cross-connect cost and size.
FIG. 4A shows a conventional domain 40 having four electrical cross-connect nodes or gateways. Each electrical cross-connect 41-44 is connected to an OC-48 transport line terminating equipment (LTE) 45 via OC-12 optical connections 46. Communication traffic enters and exits domain 40 only through the gateway cross-connects 41-44. Typically, low speed electrical communication traffic is input to the gateway cross-connect and multiplexed and converted into higher speed optical signals. For example, cross-connect 41 represents a broadband SONET digital cross-connect switch (DACS) receiving STS-1/DS-3 type traffic. Multiple electrical signals are then multiplexed, converted to optical form, and routed through the DACS 41 as shown to OC-12 optical connections 46. The OC-12 signals can then be multiplexed to OC-48 signals allowing an even greater volume of data to be transported at even higher bit rates over the working optical pipe ring 47 (or alternatively the spare optical pipe ring 48). Conversely, when traffic is to be output from a gateway cross-connect, the traffic is demultiplexed and converted back to low speed electrical signals STS-1/DS-3 type traffic.
Under normal conditions, communication traffic within domain 40 is carried on an outside optical pipe ring 47, while an inside optical pipe ring 48 is composed of dedicated spare capacity held in reserve for restoration purposes. The spare capacity is not connected for passing traffic through each node 41-44 under normal operating conditions.
FIG. 4B shows domain 40 having a fiber cut 49 breaking optical pipe rings 47 and 48 between gateway electrical cross-connects 42 and 43. These electrical cross-connects 42 and 43 on either side of break 49 respond by rerouting the communication traffic in the outside working ring 47 into the inside ring 48. Re-routing is accomplished only through complex provisioning at the electrical circuit signal level. Moreover, every traffic-bearing and spare circuit must enter and exit the cross-connect for domain restoration. This makes the domain architecture unacceptable because of the increased cross-connect size, cost, and complexity for restoration. Because all circuits--traffic-bearing and spare must enter and exit the cross-connect--this architecture is further undesirable as it precludes the use of express optical pipes through a node.
What is needed is optical domain restoration system and method for restoring a failure of an optical pipe within the same optical domain in which the failed optical pipe is located. An all-optical transmission-based restoration system and method is needed for working and spare optical rings which optimizes the use of spare capacity and restores optical communication signals through optical switching without breaking signals down to an electrical circuit level and precluding the use of express pipes.